This invention relates to improvements in electroacoustic transducers of the type used to transmit and receive pulses of pressure waves in ultrasonic equipment. Such transducers are generally comprised of an array of piezoelectric crystals mounted in parallel spaced relationship on the surface of a base of sound-absorbing material having an acoustic impedance matching that of the crystals. The base may be formed by compression-molding a composition of an acoustically high impedance material, such as tungsten powder, and an acoustically absorbing binder, such as poly vinyl chloride. In order to increase the energy absorption of the base by Rayleigh scattering, the tungsten particles are made rather large in diameter, usually greater than one-tenth the wavelength of the sound to be attenuated. In constructing such a transducer, it is customary to adhere a large crystal to the surface of the base and saw through it in parallel spaced planes so as to form the separate crystals of the array. In accordance with an invention by Amin Hanafy, set forth in his U.S. patent application Ser. No. 083,693, filed on Oct. 11, 1979, now U.S. Pat. No. 4,277,712, and entitled "Acoustic Electric Transducer with Slotted Base," harmful coupling between the crystals of the array by surface waves is substantially reduced by extending the cuts into the base. This has proven difficult to do because the saw blades frequently break when attempting to cut into the large tungsten particles referred to.
Furthermore, in order to avoid reflection at the interface of the base and the crystals and to avoid using a thick layer of adhesive in attaching the crystals to the base, it is desirable that the surface of the base to which the crystals are adhered be smooth and uniform as pointed out in U.S. patent application Ser. No. 052,705, filed June 28, 1979, in the name of J. Fleming Dias and entitled "Acoustic Imaging Transducer." The surface is prepared by polishing, but it has been found that the tungsten particles can be pulled entirely out of suitable energy absorbing binders such as poly vinyl chloride so as to leave a rough surface filled with small craters which cause undesired reflections of acoustic energy.